Tire non-uniformity relates to the symmetry (or lack of symmetry) relative to the tire's axis of rotation in mass, geometric or stiffness characteristics. Conventional tire building methods unfortunately have many opportunities for producing non-uniformities in tires. During rotation of the tires, non-uniformities present in the tire structure produce periodically-varying forces at the wheel axis. Tire non-uniformities are important when these force variations are transmitted as noticeable vibrations to the vehicle and vehicle occupants. These forces are transmitted through the suspension of the vehicle and may be felt in the seats and steering wheel of the vehicle or transmitted as noise in the passenger compartment. The amount of vibration transmitted to the vehicle occupants has been categorized as the “ride comfort” or “comfort” of the tires.
Tire uniformity characteristics, or attributes, are generally categorized as dimensional or geometric variations (radial run out (RRO) and lateral run out (LRO)), mass variance, and rolling force variations (radial force variation, lateral force variation and tangential force variation, sometimes also called longitudinal or fore and aft force variation). Uniformity measurement machines often calculate the above and other uniformity characteristics by measuring force at a number of points around a tire as the tire is rotated about its axis.
Many different factors can contribute to the presence of non-uniformities in tires, even when the tires are built under seemingly identical process conditions. Examples of such factors include the location of product start points and/or joint overlap locations for one or more of the many complex tire building products and/or steps. Exemplary products include the casing textile plies, the belt plies, bead rings, the inner liner, the tread and other rubber layers. Steps involving these and other products include the application of such products to a form or drum, placing the resulting green structure in a mold or press and subjecting the structure to heat and pressure to shape and cure the rubber products and bond the materials into an integrated unit.
The contribution of selected factors to overall tire uniformity can be modeled using vector statistics. For example, a measurement machine can be used to obtain a waveform representative of tire uniformity, which can be decomposed into respective harmonic components. Each harmonic component waveform can be characterized as a vector having some magnitude and phase, where the magnitude or length of the vector is the peak-to-peak value of the harmonic waveform and the vector phase corresponds to the angle where the first peak of the harmonic waveform occurs.
As previously mentioned, even when tires are built under seemingly identical process conditions, there will be some variation in the population of vectors. As such, it is useful to obtain a population of uniformity vectors by measuring and decomposing the uniformity harmonics for a plurality of tires. The population of vectors can then be averaged to obtain a mean vector representative of the average uniformity value for a population of tires. Known techniques for improving tire uniformity have typically been implemented to optimize this average or mean value, for example, by reducing the magnitude of the resultant mean uniformity vector. This optimization is based on the assumption that each product and/or process contributes some non-uniformity to the tire that combines to form a resultant sum. By changing the angular placement of a product or process effect in the overall tire building process, tire component effects can offset one another to reduce the average or mean value of a measured uniformity parameter.
Methods for improving tire uniformity that only optimize the average or mean value of tire uniformity ignore other meaningful statistical properties. The present disclosure provides new techniques for improving tire uniformity based on the discovery that uniformity dispersion is one such key contribution to tire uniformity modeling. Dispersion is the scattered variation of individual uniformity vectors around the average or mean vector. By optimizing dispersion levels, alone or in combination with optimization of other parameters such as the uniformity mean, improved optimization results can be achieved in accordance with aspects of the present invention.
Although known technologies for tire uniformity improvement have been developed, no design has emerged that generally encompasses all of the desired characteristics as hereafter presented in accordance with the subject technology.